MAZATLAN UNIVALVES b41 
doubt, simply because the Mazatlan shells were not numerous 
enough to oppose, while they did not confirm, his arrangement. 
While in Cerithiopsis, the sculpture is tolerably constant, in 
these, as in the British species, the rows of nodules and the 
convexity of the spire outlines vary in arrangement on the 
same specimen according to age. The basal sculpture does 
not appear in the young shell. 
391. TRIFORIS ALTERNATUS, C. B. Ad. 
Pan. Shells, no. 207, pp. 158, 310. 
Shell with the first 7 whirls transparent, light horn-coloured ; 
the first two smooth, then with numerous radiating lines, 
crossed by first one, then two, then three spiral ridges, which 
gradually become nodulous. Then of a purplish brown, for 
many whirls with two rows of stout, lighter-coloured nodules. 
Between these a fine line appears which gradually develops 
into first a smaller and at last an equal sized row of nodules. 
Below are one keel on the periphery and two on the base. 
The colour changes to whitish on the lower portion of the later 
whirls. The shell is comparatively large, a fragment measur- 
ing *08 across. A specimen with 9 normal whirls measures 
long. °17, long. spir. ‘14, lat. ‘055, div. 20°. 
Hab.— Panama, 5 sp. C. B. Adams.—Mazatlan ; portions of 
8 sp. off Spondylus ; L’pool Col. 
Tablet 1614 contains a very young sp., a perfect adolescent 
one, and a fragment of an adult. 
392. TRIFORIS INcoNnsPicuUUs, C. B. Ad. 
Pan. Shells, no. 208, pp. 159, 310. 
Shell in sculpture closely resembling T. alternatus, but much 
smaller; with the nuclear whirls shorter in proportion, light 
coloured only at the apex, developing the nodules and the dark 
colour earlier and more gradually. Colour dark throughout ; 
the last two whirls with 3 complete rows of nodules. Anterior 
spiral ribs larger, that on the periphery sometimes slightly 
sculptured. A specimen very perfect at the mouth but with 
the apex deficient, measures long. °15, long. spir. °115, lat. 05, 
div. 18°. 
Hab.—Panama; 16 sp. under stones near low water mark ; 
C. B. Adams.—Mazatlan ; 12 sp. more or less complete, off 
Spondylus and Chama; L’pool Col. 
